SocialPostStatusSyncer::applyPosted() fills the new posted_at_per_channel
JSON column on dashed__social_posts with one ISO timestamp per successfully
posted channel slug. Channels listed in errors are skipped.

Used by dashed-marketing's SocialPostResource Result section to render
per-channel posted-at + URL. Requires dashed-marketing v4.16.0+ for the
migration that adds posted_at_per_channel.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>